"Save code now" history links lead to an error (because timestamp is null)

Steps:

1. got to https://archive.softwareheritage.org/save/#requests
2. click on the URL of any succeeded task
3. it leads to an URL like: https://archive.softwareheritage.org/browse/origin/directory/?origin_url=https://github.com/MITDDC/zork.git&timestamp=null

which errors with BadInputExc: Unable to parse date string 'null'

If I'm reading Sentry correctly, it appears in v0.0.249

Sentry issue: SWH-WEBAPP-12X

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Migrated from T2557 (view on Phabricator)